A commercial aircraft usually includes an environmental control system for providing a stream of cooled, conditioned air to an aircraft cabin. A typical environmental control system receives compressed air such as bleed air from a compressor stage of an aircraft gas turbine engine, expands the compressed air in a cooling turbine and removes moisture from the compressed air via a water extractor.
Toxic ozone in the compressed air becomes an issue when an aircraft is cruising at altitudes that exceed 20,000 feet. To reduce the ozone to a level within satisfactory limits, the environmental system is provided with an ozone-destroying catalytic converter.
There are a number of desirable characteristics for an ozone-destroying catalytic converter of an aircraft. These characteristics include a) high efficiency of ozone conversion at bleed air operating temperature; b) good poison resistance from humidity, sulfur compounds, oil, dust, and the like, which may be present in the compressed air (for long life and minimum system overhaul and maintenance costs); c) light weight to minimize system parasitic load; d) high structural integrity of catalyst support under extreme heat and/or vibration shock, which may arise during normal flight conditions (also for long life and minimum system overhaul and maintenance costs); and e) high mass transport efficiency with low pressure drop.
Among the various ways known to eliminate ozone contamination from an air stream by decomposition of the ozone into oxygen are catalytic substances including metallic and non-metallic catalysts. Although catalytic systems appear to be the most efficient way to remove ozone from the bleed air or pneumatic ducting air supply system of an aircraft, many of such catalytic substances e.g. in the form of pellets or particles, must be carried in a container or canister, which creates problems with regard to weight as well as affecting the air flow. These materials also have other disadvantages. Thus, for example, the use of a catalyst coating on a metal substrate is sacrificial, in that the resultant catalyst coated oxide is readily removed and lost in the air stream. Although nickel is satisfactory it requires a very clean surface for deposition of the metal.
Many systems are known in the art for the removal of ozone from air, including those disclosed in the following U.S. patents. U.S. Pat. No. 5,422,331, incorporated herein by reference, discloses methods and catalyst compositions for abating noxious substances, particularly ozone, contained in air. The treatment of carbon monoxide, hydrogen sulfide and hydrocarbons is also discussed. A primary focus of this patent is methods of treating air taken into and/or circulated in aircraft cabins, with the cabins of trains, buses and other vehicles being mentioned as well. The patent also indicates that the disclosed catalysts can be used to abate ozone in equipment, such as xerographic copy machines, which generate ozone. Further, the patent indicates that the catalysts can be applied to surfaces in air handling systems for residences, office and factory buildings, public buildings, hospitals and the like. For this method, the catalyst can be applied to existing substrates of the air handling system, such as fan blades in air handling fans or compressors, grills, louvers or any other surface exposed to the air stream.
U.S. Pat. No. 4,206,083, incorporated herein by reference, discloses the co-precipitation of platinum, palladium and manganese oxide on a ceramic support, such as a cordierite support, in order to provide a catalyst suitable for the reduction of ozone content of air intended for human respiration. The patent refers to U.S. Pat. No. 3,269,801 as evidence that it had been long recognized that ozone is present in the atmosphere in toxic concentrations at high altitudes. Aircraft flying at those altitudes scoop in cabin air from the outside atmosphere which, because it is very much compressed, is raised in temperature to several hundred degrees centigrade. Such air is treated to reduce the ozone concentration of it to below 1 part per million (“ppm”) to render it fit for use as cabin air.
U.S. Pat. No. 5,187,137, incorporated herein by reference, discloses an ozone abatement catalyst comprising a composition containing manganese oxide and metallic palladium and/or palladium oxide as the essential ingredients, formed as a thin film on a support. The method of preparing the catalyst includes coating on a support a slurry containing manganese oxide, metallic palladium and/or a palladium compound, and an inorganic oxide sol as a binder, for example, an alumina sol, silica sol, titania sol or zirconia sol. The patent discloses that the support, i.e., the substrate on which the catalytic material is disposed, may be a cordierite or other similar inorganic support, or it may be a metal support.
U.S. Pat. No. 4,900,712, incorporated herein by reference, discloses a catalytic washcoat in which one or more catalytically active non-noble metal oxides (“dopants”) such as iron oxide are deposited from neutral aqueous colloidal solutions thereof onto preformed high surface area crystalline alumina. The neutral colloidal compounds are said to provide a uniform, thin coating of the non-noble dopants on the alumina particles and to avoid the use of noxious elements such as nitrates or chlorides, thereby substantially eliminating any air pollution hazard. The reference to eliminating air pollution hazards appears to be with reference to the manufacture of the catalyst.
U.S. Pat. No. 4,343,776, incorporated herein by reference, discloses an ozone abatement catalyst containing at least one platinum group metal (platinum, palladium or rhodium) or catalytically active compound thereof and an oxide or aluminate of at least one non-precious Group VIII metal (iron, cobalt or nickel). By applying the non-precious metal oxide as an alumina slip prior to application of the platinum group metal component to the substrate, the platinum group metal, e.g., palladium, is preferentially exposed to the ozone. The carrier or support, i.e., the substrate, may be any one of a wide range of materials, including aluminum.
U.S. Pat. No. 5,250,489, incorporated herein by reference, discloses a catalyst structure configured to provide heat exchange in which the support, i.e., substrate, is a metallic support which may comprise aluminum or aluminum alloys, provided the latter are used at temperatures which will not deform or melt the material. However, other materials, including aluminum-containing steels are described.
U.S. Pat. No. 5,080,882, incorporated herein by reference, discloses an ozone decomposition catalyst disposed on a thin porous carrier (substrate) material which has micropores of preferably not less than 30 microns in diameter for ozone abatement, in order to prevent substantial pressure loss. Any suitable known ozone catalyst may be utilized, including oxides of manganese, iron, silver, nickel or copper and a noble metal such as platinum or palladium or a mixture of two or more of these.
In addition to the problems caused by elevated levels of ozone in aircraft air, high levels of volatile organic compounds (VOCs) can also cause substantial passenger discomfort. The treatment of aircraft intake gases containing VOCs has been of increasing concern in recent years. Catalytic oxidation and adsorption are commonly used for removing these pollutants. In some instances, adsorption by adsorbents such as carbon can be used; however, this process does not destroy the pollutants, but merely concentrates them. Furthermore, adsorption efficiency can be adversely impacted by fluctuating concentrations of the gaseous components. Catalytic oxidation is a more energy efficient and economical way of destroying VOCs in air intake systems. Catalytic oxidation operates at significantly lower temperatures and requires smaller reactors made of less expensive materials.
Methods for the catalytic oxidation of VOCs are well known in the art. For example, U.S. Pat. Nos. 3,972,979 and 4,053,557, incorporated herein by reference, describe the decomposition of halogenated hydrocarbons by oxidation over chromium oxide or a boehmite supported platinum.
U.S. Pat. Nos. 4,059,675, 4,059,676 and 4,059,683, incorporated herein by reference, describe methods for decomposing halogenated organic compounds using catalysts containing ruthenium, ruthenium-platinum and platinum, respectively, in the presence of an oxidizing agent at a temperature of at least 350° C.
U.S. Pat. No. 5,283,041, incorporated herein by reference, discloses an oxidation catalyst for treating a gas stream containing compounds selected from the group consisting of halogenated organic compounds, other organic compounds and mixtures thereof; the catalyst comprising a core material comprising zirconium oxide and one or more oxides of manganese, cerium or cobalt with vanadium oxide and, preferably, platinum group metal dispersed on the core material.
U.S. Pat. No. 5,643,545, incorporated herein by reference, relates to treatment of streams containing halogenated organic compounds and volatile organic compounds (VOCs) with catalytic materials deposited on high acidity and/or low acidity supports.
U.S. Pat. Nos. 5,578,283 and 5,653,949, incorporated herein by reference, relate to treatment of gases containing halogenated organic compounds, non-halogenated organic compounds, carbon monoxide or mixtures thereof. Catalyst compositions useful in the treatment comprise at least one platinum group metal, zirconium oxide and at least one oxide of manganese, cerium or cobalt. A further composition disclosed uses the foregoing described components but which is substantially free of vanadium in a process for treating a gas stream containing at least one brominated organic compound.
U.S. Pat. No. 6,319,484, incorporated herein by reference, discloses a composition for abatement of airborne pollution by volatile organic compounds (“VOCs”), having an upstream composition containing a protective adsorbent, e.g., Y zeolite, which is effective for adsorbing large VOC molecules. The downstream composition contains a second adsorbent, e.g., a silver-containing ZSM-5, which is effective for adsorbing relatively smaller VOC molecules, e.g., propylene, and a second oxidation catalyst intimately intermingled therewith. Oxidation of VOCs while they are still retained on the adsorbents is promoted at temperatures lower than would be required if the VOCs were desorbed into the gaseous phase.
However, all of the ozone and VOC converters have limitations. Specifically, they have relatively short useful life spans. Currently, aircraft original equipment manufacturers look to regenerate or replace converters every 4,000 to 15,000 flight miles in some applications. Extending the life of these converters could result in significant savings. The short life spans of currently used catalysts result from premature poisoning of the catalysts by inorganic species of zinc, phosphorus, calcium, and other elements, which may leak into the air stream from the lube oil additives used in the engine and/or compressor.
Therefore, it is an object of the present invention to provide an apparatus for improved purification of air, particularly aircraft cabin air. It is another objective of the present invention to provide a means of removing inorganic compounds and particulate matter from inlet gas streams, thereby preventing catalytic poisoning, and thereby extending the life of the catalysts.